In recent years, portable electronic devices such as notebook personal computers and cellular phones have been variously functionalized as communication functions, movie reproduction functions, camera functions and the like. For power storage devices to be used for these portable electronic devices, there are demanded a high energy density, a high output characteristic, a high rate characteristic, a high safety and a high cycle stability.
As a power storage device having a high output characteristic and a high rate characteristic, there is proposed a power storage device (hereinafter, referred to as “organic radical battery”) containing a nitroxyl compound in its cathode (for example, Patent Literature 1). The nitroxyl compound takes an oxoammonium cation substructure in an oxidized state and takes a nitroxyl radical substructure in a reduced state, and carries out electron transfer between the two states; and this reaction is used as an electrode reaction of the cathode. Since the electrode reaction progresses relatively rapidly, the power storage device is allowed to be discharged and charged at large currents. That is, there can be obtained a secondary battery having a high output, and the “organic radical battery” posing no problem of thermal runaway is a secondary battery having high safety. In recent years, however, needs for quick charging have risen and a further high rate characteristic has been demanded.
Patent Literature 2 discloses an ionic conductive material containing an ionic compound represented by the formula: Li+[(FSO2)2N] in a solution of an aprotic solvent. It is disclosed that the [(FSO2)2N]− anion (hereinafter, FSI anion) has wide stabilities against the redox phenomenon and has a high conductivity. Further Patent Literature 3 discloses a lithium secondary battery characterized in using an electrolyte solution containing FSI anions in the lithium secondary battery in which a lithium transition metal composite oxide is employed as its cathode. In this battery, it is conceivable that by using a specific electrolyte solution containing FSI anions, a film is formed on the electrode and a rise in the internal resistance is suppressed to thereby realize a high rate characteristic. Further Patent Literature 4 reports that by using, as an electrolyte solution for a lithium secondary battery, an electrolyte solution containing acetonitrile being a low-viscosity solvent and Li+[(FSO2)2N]− of 3.0 mol/L or higher, an excellent rate characteristic is attained.